1. Field of the Invention
The present invention relates to a semiconductor chip, and more particularly, it relates to the structure of a semiconductor chip suitable for a chip-on-board system and methods of fabricating and mounting the same.
For example, a semiconductor chip such as an IC chip provided with a plurality of terminals on its major surface is mounted on a printed wiring board or a lead frame in the so-called chip-on-board system. In order to cope with the chip-on-board mount system, an anisotropic conductive film, for example, is stuck on the major surface of the semiconductor chip.
2. Description of the Prior Art
FIG. 1 illustrates a semiconductor chip 10 which is mounted on a circuit board 20 in the chip-on-board system. A plurality of first bumps 11 for connection are protrusively formed on a major surface of the semiconductor chip 10. On the other hand, a plurality of second bumps 21 are protrusively formed on a surface of the circuit board 20, to be opposed to the first bumps 11 of the semiconductor chip 10 respectively. The chip-on-board system is adapted to oppose the major surface of the semiconductor chip 10 to the circuit board 20 through an anisotropic conductive film 30 and bring these members into pressure contact with each other under heat, thereby electrically connecting and bonding the opposed first and second bumps 11 and 21 with each other while keeping insulation in the remaining regions. Throughout the specification, the term xe2x80x9cbumpxe2x80x9d must be understood as indicating an electrode, a terminal pad or a conductor pad formed in a slightly protruding manner.
The anisotropic conductive film 30 includes an adhesive resin film 31 and conductive grains 32 dispersed in this resin film 31. The conductive grains 32 are prepared from metal balls, for example. The metal balls may be replaced with resin balls having nickel-plated surfaces or those further having gold plating on such nickel-plated surfaces, for example.
When prescribed pressure is so applied under heat that the semiconductor chip 10 and the circuit board 20 approach to each other through the anisotropic conductive film 30 interposed therebetween, portions of the anisotropic conductive film 30 located between the first bumps 11 of the semiconductor chip 10 and the second bumps 21 of the circuit board 20 are softened and crushed. Consequently, the first and second bumps 11 and 21 are electrically connected with each other through the conductive grains 32.
In the remaining uncrushed regions of the anisotropic conductive film 30 not located between the first and second bumps 11 and 21, the conductive grains 32 are still dispersed. Thus, these regions keep insulation. Further, the semiconductor chip 10 and the circuit board 20 are bonded with each other due to the adhesive strength of the anisotropic conductive film 30.
In the aforementioned mount method, electrical conduction can be attained only in necessary portions, and the semiconductor chip 10 can be mounted on the circuit board 20 by simply applying pressure so that the semiconductor chip 10 and the circuit board 20 approach to each other through the anisotropic conductive film 30 interposed therebetween. This method is remarkably simple as compared with a method of mounting the semiconductor chip 10 on the circuit board 20 or the like by the so-called chip bonding and wire bonding.
However, the method of connecting the bumps 11 and 21 through the anisotropic conductive film 30 shown in FIG. 1 is merely adapted to press the semiconductor chip 10 and the circuit board 20 through the anisotropic conductive film 30 interposed therebetween with application of heat. Therefore, this method is insufficient. in stability of electrical connection. Namely, the conductive grains 32 contained in the anisotropic conductive film 30 may not properly come into contact with the bumps 11 and 21 unless the heating temperature and the pressure-contact force are properly managed. When the semiconductor chip 10 is to be mounted on a printed circuit board based on glass epoxy resin or the like, the board may be distorted or warped, leading to loose connection between the bumps 11 and 21.
In the conventional mount method shown in FIG. 1, the anisotropic conductive film 30 having a size corresponding to that of the semiconductor chip 10 is previously prepared, to be interposed between the semiconductor chip 10 and the circuit board 20. The prepared anisotropic conductive film 30 is first placed on a prescribed position of the circuit board 20, so that the semiconductor chip 10 is placed on this anisotropic conductive film 30. Alternatively, the anisotropic conductive film 30 formed in a prescribed size is previously stuck on the major surface of the semiconductor chip 10, so that the semiconductor chip 10 having the anisotropic conductive film 30 is placed on a prescribed position of the circuit board 20.
In either case, a single anisotropic conductive film 30 must be prepared for a single semiconductor chip 10, in order to mount the semiconductor chip 10 on the circuit board 20 in the chip-on-board system. Therefore, the anisotropic conductive film 30 must be prepared in the size corresponding to that of the semiconductor chip 10, and such anisotropic conductive films 30 must be prepared in a number responsive to that of semiconductor chips 10 to be mounted, leading to reduction of workability and increase in cost. Further, the anisotropic conductive films 30 formed in the prescribed size must be placed one by one on the circuit boards 20 or stuck one by one to the surfaces of the semiconductor chips 10. Thus, the number of working steps is increased and the workability is deteriorated.
In general, each semiconductor chip 10 is packaged with resin, to be prevented from penetration of moisture or air from the exterior. If an unpackaged semiconductor chip 10 is mounted on the circuit board 20 or the like in the chip-on-board system, an electronic circuit formed on the semiconductor chip. 10 cannot be completely blocked and shielded against the exterior in the mounted state. Therefore, moisture or air disadvantageously penetrates into the electronic circuit of the semiconductor chip 10, to inhibit the semiconductor chip 10 from exhibiting its original characteristics.
An object of the present invention is to further ensure electrical connection between butted terminals.
Another object of the present invention is to improve workability in case of mounting a semiconductor chip on a circuit board or the like in the chip-on-board system.
Still another object of the present invention is to effectively prevent penetration of moisture or air from the exterior in a semiconductor chip having an anisotropic conductive film stuck on its major surface.
A connection structure between terminals according to an aspect of the present invention comprises a first member having a first terminal on its surface, a second member having a second terminal opposed to the first terminal on its surface, and an anisotropic conductive film arranged between the first and second members. The anisotropic conductive film includes an adhesive resin film and conductive grains dispersed in this resin film. The first and second terminals are electrically connected with each other through the conductive grains. The. conductive grains are alloy-bonded with at least either one of the first and second terminals. Typically, the first member is a semiconductor chip, and the second member is a circuit board or a lead frame.
No alloy bonding is attained by merely heating/pressing the anisotropic conductive film. Namely, the alloy bonding can be implemented by properly selecting metals forming the terminals and the conductive grains contained in the anisotropic conductive film and supplying ultrasonic vibration between the terminals to be electrically connected with each other. The alloy bonding highly attains stability of the electrical connection between the first and second terminals.
A method of connecting terminals according to another aspect of the present invention comprises steps of arranging a first terminal formed on a surface of a first member and a second terminal formed on a surface of a second member to be opposed to each other, interposing a film between the opposed first and second members, and supplying ultrasonic vibration between the first and second terminals while applying pressure so that the first and second members approach to each other thereby electrically connecting the first and second terminals with each other. In an embodiment, the film is an anisotropic conductive film prepared by dispersing conductive grains in an adhesive resin film. In another embodiment, the film is a thermo-adhesive resin film. Typically, the first member is a semiconductor chip, and the second member is a circuit board or a lead frame.
The anisotropic conductive film is basically identical to a conventional one employed in the chip-on-board system. Namely, the conductive film has a structure dispersing conductive grains in an adhesive resin film. The conductive grains, which are dispersed in the resin film, are isolated from each other by insulating resin in a general state. In this case, therefore, both surfaces of the anisotropic conductive film are insulated from each other. When pressure is applied to a prescribed region of the anisotropic conductive film along its thickness preferably under heat to crush the film, on the other hand, surfaces of the conductive grains are exposed on both surfaces of the film which is crushed and reduced in thickness. Therefore, the surfaces opposed to each other through the grains electrically conduct with each other.
According to the present invention, not only the anisotropic conductive film interposed between the first and second terminals is pressed but ultrasonic vibration is applied between the terminals for electrically connecting the same with each other. Preferably, the-first and second terminals and the anisotropic conductive film interposed therebetween are heated. When ultrasonic vibration is supplied between the first and second terminals the conductive grains contained in the anisotropic conductive film crushed therebetween are reliably electrically connected with the terminals by vibrational friction. In addition, the metals forming the terminals and the conductive grains are so properly selected as to form a eutectic alloy on contact portions thereof. Formation of such a eutectic alloy leads to further reliable electrical connection between the terminals and the conductive grains.
The surfaces provided with the first and second terminals respectively are fixed to each other due to bonding through the anisotropic conductive film. Consequently, stability of the electrical connection between the terminals is improved according to the inventive method of connecting terminals, as compared with the conventional method employing an anisotropic conductive film. This effect also applies to the case of mounting a semiconductor chip on a board by a chip-on-board mount method or the case of bonding the semiconductor chip on a lead frame. When ultrasonic vibration is supplied under pressure with interposition of the anisotropic conductive film, the terminals are preferably heated. However, it has been confirmed that the temperature is increased by vibrational friction to substantially thermally bond the surfaces provided with the terminals with each other by simply applying pressure and ultrasonic vibration with no heating, by properly selecting the energy.
In case of interposing not an anisotropic conductive film but a thermo-adhesive resin film between the first and second terminals, the resin located between the convexly formed first and second terminals is heated and softened by internal friction caused by ultrasonic vibration between the terminals and quickly flows sidewise to extrude, thereby attaining direct contact between the first and second terminals. The surfaces provided with the first and second terminals respectively are bonded and fixed to each other by the adhesive strength of the resin.
According to the aforementioned method, the surfaces provided with the opposed terminals can be readily connected with each other while attaining electrical conduction of the opposed terminals and insulating the remaining regions from each other by simply performing the aforementioned operation with interposition of an insulating resin sheet of epoxy resin, for example, without employing an anisotropic conductive film in particular. This effect also applies to the case of mounting a semiconductor chip on a substrate by a chip-on-board mount method or the case of bonding the semiconductor chip on a lead frame. While the terminals are preferably heated when pressure and ultrasonic vibration are applied therebetween as hereinabove described, heat may not necessarily be supplied from the exterior. Namely, it has been confirmed that the temperature is increased by vibrational friction to substantially thermally bond the surfaces provided with the terminals with each other by simply supplying ultrasonic vibration and properly selecting the energy.
A method of fabricating a semiconductor chip according to still another aspect of the present invention comprises steps of forming a plurality of circuit elements on a major surface of a semiconductor wafer, forming bumps on electrode pads of the circuit elements, forming an anisotropic conductive film covering the overall major surface of the semiconductor wafer, and splitting the semiconductor wafer into each circuit element thereby obtaining a semiconductor chip having an anisotropic conductive film on its surface.
According to the aforementioned fabrication method, the anisotropic conductive film is stuck not on each semiconductor chip but on the wafer not yet split into each semiconductor chip. Thus, the anisotropic conductive film can be simultaneously stuck on all semiconductor chips obtained from a single wafer.
According to the aforementioned method, the anisotropic conductive film may not be prepared in a size corresponding to that of each semiconductor chip, and there is no need to prepare anisotropic conductive films in a number responsive to that of the semiconductor chips for mounting each semiconductor chip on a circuit board or the like. Thus, the workability is improved and the cost can be reduced. Further, the method requires no operation of placing anisotropic conductive films formed in a prescribed size one by one on prescribed positions of circuit boards or the like or no operation of sticking such anisotropic conductive films one by one on surfaces of semiconductor chips, whereby the workability is improved.
Preferably, the step of obtaining the semiconductor chip includes steps of bonding the semiconductor wafer on an expandable tape, forming slits on the anisotropic conductive film and the semiconductor wafer along the boundary lines between the plurality of circuit elements, and splitting the semiconductor wafer into a plurality of semiconductor chips by expanding the expandable tape.
According to the aforementioned preferred method, the semiconductor chips are regularly arranged on the expandable tape at prescribed intervals. Therefore, the semiconductor chips can be directly placed on circuit boards or the like from the state bonded on the expandable tape with the so-called chip mounter or the like, for example. Thus, a series of operations for fabricating the semiconductor chips, sticking the anisotropic conductive film, and placing the semiconductor chips on circuit boards or the like can be simplified.
Provided according to a further aspect of the present invention is a method of mounting a semiconductor chip having a bump on its major surface on a support member having a conductor pad to be connected with the bump. This method comprises steps of sticking an anisotropic conductive film on the overall major surface of the semiconductor chip, placing the semiconductor chip on the support member so that the bump and the conductor pad are opposed to each other through the anisotropic conductive film, and electrically connecting the bump and the conductor pad with each other by applying pressure so that the semiconductor chip and the support member approach to each other under heat. Preferably, ultrasonic vibration is supplied between the semiconductor chip and the support member when applying the pressure. The support member is typically a circuit board or a lead frame.
A semiconductor chip structure according to a further aspect of the present invention comprises a semiconductor chip having a bump for electrical connection on its major surface, and an anisotropic conductive film which is stuck on the semiconductor chip to cover the overall major surface of the semiconductor chip and has an outer peripheral edge protruding outward beyond that of the semiconductor chip.
When the aforementioned semiconductor chip is pressed in a heated state to be mounted on a circuit board or the like, a resin film of the anisotropic conductive film is so softened that the semiconductor chip engages into the softened resin film. In other words, the side surface of the semiconductor chip is covered with the resin film of the anisotropic conductive film, whereby an electronic circuit of the semiconductor chip is completely blocked and shielded against the exterior. In particular, the anisotropic conductive film is stuck on the semiconductor chip to protrude beyond the peripheral edge thereof in the aforementioned semiconductor chip structure, whereby the side surface of the semiconductor chip is further reliably covered with the resin film of the anisotropic conductive film.
When the side surface of the semiconductor chip is covered with the resin film of the anisotropic conductive film, an effect similar to that in case of packaging the semiconductor chip with resin is attained. Namely, the electronic circuit of the semiconductor chip can be prevented from penetration of moisture or air through the resin film of the anisotropic conductive film with no resin packaging, and the semiconductor chip can sufficiently exhibit its original characteristics utilized.
According to a preferred embodiment, the semiconductor chip structure further comprises a protective film covering a corner part between the side surface of the semiconductor chip and the rear surface of the protruding part of the anisotropic conductive film. The protective film is formed along the side surface of the semiconductor chip. When the semiconductor chip is mounted on a circuit board or the like, therefore, the electronic circuit is blocked and shielded against the exterior by the protective film, in addition to the softened resin film of the anisotropic conductive film. Thus, the aforementioned effect can be further prompted.
In a method of mounting a semiconductor chip according to a further aspect of the present invention, an anisotropic conductive film is first stuck on the overall major surface of a semiconductor chip. The outer peripheral edge of the anisotropic conductive film protrudes outward beyond that of the semiconductor chip. Then, the semiconductor chip is placed on a support member so that a bump of the semiconductor chip and a conductor pad of the support member are opposed to each other through the anisotropic conductive film. Thereafter pressure is applied under heat so that the semiconductor chip and the support member approach to each other, for electrically connecting the bump and the conductor pad with each other.
When pressure is applied to the anisotropic conductive film along its thickness in a heated state to crush the same in the aforementioned mount method, a resin film of the softened anisotropic conductive film extends along the side surface of the semiconductor chip. When the resin film is hardened and the semiconductor chip is mounted, an electronic circuit of the semiconductor chip is blocked and protected against the exterior by the resin film. When the semiconductor chip provided with a protective film along its side surface is mounted on a circuit board or the like, the electronic circuit of the semiconductor chip can be further reliably protected.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.